Exhaust Purification System of Internal Combustion Engine

ABSTRACT

An engine exhaust passage is provided with a fuel addition valve ( 14 ) for adding fuel in the form of atomized drops, an oxidation catalyst ( 11 ), and an NO x  storage and reduction catalyst ( 12 ) in that order. The oxidation catalyst ( 11 ) carries as a precious metal not only platinum Pt, but also palladium Pd. A molar ratio of the platinum Pt with respect to the sum of the platinum Pt and palladium Pd carried on the oxidation catalyst ( 11 ) is set to between about 50 percent to about 80 percent.

TECHNICAL FIELD

The present invention relates to an exhaust purification system of aninternal combustion engine.

BACKGROUND ART

Known in the art is an internal combustion engine provided with anNO_(x) storage and reduction catalyst storing the NO contained inexhaust gas when the air-fuel ratio of the exhaust gas flowing into theengine exhaust passage is lean and releasing the stored NO_(x) when theair-fuel ratio of the inflowing exhaust gas becomes the stoichiometricair-fuel ratio or rich. This NO_(x) storage and reduction catalystincludes a precious metal catalyst comprised of platinum Pt and anNO_(x) absorbent. When the air-fuel ratio is lean, the NO_(x) containedin the exhaust gas, that is, the NO, is oxidized on the platinum Pt toNO₂, then is stored in the form of nitrate ions NO₃ ⁻ in the NO_(x)absorbent.

On the other hand, when releasing and reducing the NO_(x) absorbed fromthe NO_(x) absorbent, the air-fuel ratio of the exhaust gas flowing intothe NO_(x) storage and reduction catalyst is made rich. If the air-fuelratio of the exhaust gas is made rich, the oxygen concentration in theexhaust gas falls, so the NO_(x) absorbed in the NO_(x) absorbent in theform of nitrate ions NO₃ ⁻ appears in the form of NO₂ on the platinum Ptfrom inside the NO_(x) absorbent. This NO₂ is reduced by the unburnt HCand CO contained in the exhaust gas.

Note that the air-fuel ratio of the exhaust gas is made rich bysupplying additional fuel into the combustion chamber or by addingadditional fuel into the engine exhaust passage. In this case, when theadded fuel is gaseous and flows into the NO_(x) storage and reductioncatalyst, if the air-fuel ratio of the exhaust gas is made rich, theNO_(x) is released from the NO_(x) storage and reduction catalyst andreduced. However, when the air-fuel ratio of the exhaust gas should bemade rich, additional fuel is added into the engine exhaust passage inthe form of atomized drops. The situation differs somewhat when thisadded fuel deposits in the form of drops on the NO_(x) storage andreduction catalyst.

That is, if the fuel to be added when the air-fuel ratio of the exhaustgas should be made rich deposits in the form of drops on the NO_(x)storage and reduction catalyst, the platinum Pt on the NO_(x) storageand reduction catalyst is covered by the fuel drops. However, if theplatinum Pt is covered by the fuel drops, the oxygen in the exhaust gascannot reach the surface of the platinum Pt and as a result theoxidation reaction of the fuel drops on the platinum Pt can no longer beperformed well. If the oxidation reaction of the fuel drops cannot beperformed well, the oxygen in the exhaust gas will not be sufficientlyconsumed and as a result the oxygen concentration in the exhaust gaswill not sufficiently fall, so NO_(x) will no longer be released wellfrom the NO_(x) absorbent. Further, the fuel drops will insufficientlyvaporize, so the amount of the gaseous state unburned HC contained inthe exhaust gas will not be sufficient and therefore the released NO_(x)will not be able to be sufficiently reduced.

Therefore, the inventors took note of the oxygen storing capability ofpalladium Pd in the process of their research and discovered that byusing as a precious metal not only platinum Pt, but also palladium Pd,the large amount of oxygen stored on the palladium Pd will promote theoxidation reaction of the fuel drops, the heat of this oxidationreaction will promote the vaporization of the fuel drops on the platinumPt, and therefore the NO_(x) release action from the NO_(x) absorbentwill be promoted.

In this case, if increasing the amount of palladium Pd and decreasingthe amount of platinum Pt, the heat of the oxidation reaction of theoxygen stored in the palladium Pd will promote the vaporization of thefuel drops on the platinum Pt, but due to the small amount of platinumPt, the release action of NO_(x) will be weak and as a result anexcellent NO_(x) release action will not be able to be obtained. Asopposed to this, if reducing the amount of palladium Pd and increasingthe amount of platinum Pt, the vaporization of the fuel drops on theplatinum Pt will not be sufficiently promoted due to the heat of theoxidation reaction of the stored oxygen of the palladium Pd, so even ifincreasing the amount of platinum Pt, only a weak NO_(x) release actionwill be obtained and, therefore, in this case as well, an excellentNO_(x) release action will not be obtained.

That is, the ratio of platinum Pt and palladium Pd giving an excellentNO_(x) release action exists in a ratio of suitable ratios neitherextremely large nor extremely small. Regarding this point, a knowndiesel particulate filter (see Japanese Patent Publication (A) No.2003-205245) is designed to carry platinum Pt and palladium Pd inamounts of 1 g each per liter volume of the filter body. This becomesabout 35.7 in terms of the molar ratio of platinum Pt with respect tothe sum of the platinum Pt and palladium Pd. However, with this molarratio, the amount of palladium Pd is too great compared with theplatinum Pt and as a result an excellent NO_(x) release action cannot beobtained.

DISCLOSURE OF THE INVENTION

The present invention provides a ratio of platinum Pt and palladium Pdenabling an excellent NO_(x) release action to be secured even if theplatinum Pt is covered by the fuel drops when the fuel is added in theform of drops so as to release NO_(x) from the NO_(x) storage andreduction catalyst.

That is, according to the present invention, there is provided aninternal combustion engine providing a fuel addition device adding fuelin the form of atomized drops inside an engine exhaust passage,providing an oxidation catalyst inside the engine exhaust passagedownstream of the fuel addition device, providing an NO_(x) storage andreduction catalyst storing the NO_(x) contained in the exhaust gas whenthe air-fuel ratio of the inflowing exhaust gas is lean and releasingthe stored NO_(x) when the air-fuel ratio of the inflowing exhaust gasbecomes a stoichiometric air-fuel ratio or rich inside the engineexhaust passage downstream of the oxidation catalyst, adding fuel fromthe fuel addition device when making the air-fuel ratio of the exhaustgas flowing into the NO_(x) storage and reduction catalyst rich so as tomake the NO_(x) storage and reduction catalyst release NO_(x), andmaking the fuel added at this time deposit in the form of drops on theoxidation catalyst, wherein to make the fuel deposited in the form ofdrops on the oxidation catalyst vaporize, the oxidation catalyst carriesas a precious metal not only platinum Pt, but also palladium Pd and themolar ratio of the platinum Pt with respect to the sum of the platinumPt and palladium Pd carried on the oxidation catalyst is set from about50 percent to about 80 percent.

Further, according to the present invention, there is provided aninternal combustion engine providing a fuel addition device adding fuelin the form of atomized drops inside an engine exhaust passage,providing an NO_(x) storage and reduction catalyst storing the NO_(x)contained in the exhaust gas when the air-fuel ratio of the inflowingexhaust gas is lean and releasing the stored NO_(x) when the air-fuelratio of the inflowing exhaust gas becomes a stoichiometric air-fuelratio or rich inside the engine exhaust passage downstream of theoxidation catalyst, adding fuel from the fuel addition device whenmaking the air-fuel ratio of the exhaust gas flowing into the NO_(x)storage and reduction catalyst rich so as to make the NO_(x) storage andreduction catalyst release NO_(x), and making the fuel added at thistime deposit in the form of drops on the NO_(x) storage and reductioncatalyst, wherein to make the fuel deposited in the form of drops on theNO_(x) storage and reduction catalyst vaporize, the NO_(x) storage andreduction catalyst carries as a precious metal not only platinum Pt, butalso palladium Pd and the molar ratio of the platinum Pt with respect tothe sum of the platinum Pt and palladium Pd carried on the NO_(x)storage and reduction catalyst is set from about 50 percent to about 80percent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall view of a combustion ignition type internalcombustion engine,

FIG. 2 is an overall view of another embodiment of a combustion ignitiontype internal combustion engine,

FIG. 3 are views showing a particulate filter,

FIG. 4 are views for explaining an absorption/release action of NO_(x),

FIG. 5 is a side cross-sectional view of an oxidation catalyst,

FIG. 6 are cross-sectional views illustrating a surface part of asubstrate of an oxidation catalyst,

FIG. 7 is a view showing the relationship between an oxidation speed anda platinum molar ratio,

FIG. 8 is a view showing the relationship between an NO_(x) purificationrate and a temperature of the oxidation catalyst,

FIG. 9 is a view showing the relationship between an NO_(x) purificationrate and a platinum molar ratio,

FIG. 10 is a view of a comparative example,

FIG. 11 is a time chart of NO_(x) release processing,

FIG. 12 is a view of a map of a per unit time NO_(x) storage amount,

FIG. 13 is a flowchart for exhaust purification processing, and

FIG. 14 is a view of another embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is an overall view of a combustion ignition type internalcombustion engine.

Referring to FIG. 1, 1 indicates an engine body, 2 a combustion chamberof a cylinder, 3 an electronic control type fuel injector for injectingfuel into a combustion chamber 2, 4 an intake manifold, and 5 an exhaustmanifold. The intake manifold 4 is connected through an intake duct 6 toan outlet of a compressor 7 a of the exhaust turbocharger 7, while aninlet of the compressor 7 a is connected to an air cleaner 8. Inside theintake duct 6 is provided a throttle valve 9 driven by a step motor.Further, around the intake duct 6 is provided a cooling device forcooling the intake air circulating inside the intake duct 6. In theembodiment shown in FIG. 1, engine cooling water is led inside thecooling device 10, where the engine cooling water cools the intake air.

On the other hand, the exhaust manifold 5 is connected to an inlet of anexhaust turbine 7 b of the exhaust turbocharger 7, while an outlet ofthe exhaust turbine 7 b is connected to an inlet of an oxidationcatalyst 11. Further, the outlet of the oxidation catalyst 11 isconnected through an exhaust pipe 13 to an NO_(x) storage and reductioncatalyst 12. The exhaust manifold 5 mounts a fuel addition valve 14 foradding mist-like, that is, particulates of, fuel, in the form of dropsinto the exhaust gas. In the embodiment according to the presentinvention, this fuel is comprised of diesel oil.

The exhaust manifold 5 and the intake manifold 4 are connected to eachother through an exhaust gas recirculation (hereinafter referred to asthe “EGR”) passage 15. Inside the EGR passage 15 is arranged anelectronic control type EGR control valve 16. Further, around the EGRpassage 15 is arranged a cooling device 17 for cooling the EGR gasflowing through the inside of the EGR passage 15. In the embodimentshown in FIG. 1, the engine cooling water is led to the inside of thecooling device 17 where the engine cooling water then cools the EGR gas.On the other hand, each fuel injector 3 is connected through a fuel tube18 to a common rail 19. This common rail 19 is fed with fuel from anelectronic control type variable discharge fuel pump 20. The fuel fedinto the common rail 19 is supplied through each fuel tube 18 to eachfuel injector 3.

An electronic control unit 30 is comprised of a digital computerprovided with a ROM (read only memory) 32, RAM (random access memory)33, CPU (microprocessor) 34, input port 35, and output port 36 connectedto each other by a bi-directional bus 31. The NO_(x) storage andreduction catalyst 12 is provided with a differential pressure sensor 21for detecting the pressure difference before and after the NO_(x)storage and reduction catalyst 12. The output signal of thisdifferential pressure sensor 21 is input through a corresponding ADconverter 37 to an input port 35. Further, an acceleration pedal 40 hasconnected to it a load sensor 41 generating an output voltageproportional to the amount of depression L of the acceleration pedal 40.The output voltage of the load sensor 41 is input through acorresponding AD converter 37 to the input port 35. Further, the inputport 35 has connected to it a crank angle sensor 42 generating an outputpulse each time a crankshaft for example rotates by 15°. On the otherhand, the output port 36 is connected through a corresponding drivecircuit 38 to the fuel injector 3, throttle valve 9 drive step motor,fuel addition valve 14, EGR control valve 16, and a fuel pump 20.

FIG. 2 shows another embodiment of a combustion ignition type internalcombustion engine. In this embodiment, the engine exhaust passage is notprovided with any oxidation catalyst. The outlet of the exhaust turbine7 b is connected to the inlet of the NO_(x) storage and reductioncatalyst 12.

First, explaining the NO_(x) storage and reduction catalysts 12 shown inFIG. 1 and FIG. 2, these NO_(x) storage and reduction catalysts 12 arecarried on three-dimensional net structure monolith carriers orpellet-shaped carriers or are carried on a particulate filter forming ahoneycomb structure. In this way, the NO_(x) storage and reductioncatalyst 12 can be carried on various carriers, but below, the case ofcarrying the NO_(x) storage and reduction catalyst 12 on a particulatefilter will be explained.

FIGS. 3(A) and (B) show the structure of a particulate filter 12 acarrying an NO_(x) storage and reduction catalyst 12. Note that FIG.3(A) shows the front view of the particulate filter 12 a, while FIG.3(B) shows the side cross-sectional view of the particulate filter 12 a.As shown in FIGS. 3(A) and (B), the particulate filter 12 a forms ahoneycomb structure provided with a plurality of exhaust passages 60, 61extending in parallel. These exhaust passages are comprised of exhaustgas inflow passages 60 with downstream ends sealed by plugs 62 andexhaust gas outflow passages 61 with upstream ends sealed by plugs 63.Note that in FIG. 3(A), the hatched parts show the plugs 63. Therefore,the exhaust gas inflow passages 60 and exhaust gas outflow passages 61are alternately arranged across thin partition walls 64. In other words,the exhaust gas inflow passages 60 and exhaust gas outflow passages 61are arranged so that each exhaust gas inflow passage 60 is surrounded byfour exhaust gas outflow passages 61 and each exhaust gas outflowpassage 61 is surrounded by four exhaust gas inflow passages 60.

The particulate filter 12 a is for example formed from a porous materialsuch as cordierite. Therefore, the exhaust gas flowing into the exhaustgas inflow passage 60, as shown by the arrows in FIG. 3(B), flows outinto the adjoining exhaust gas outflow passages 61 through thesurrounding partition walls 64.

When carrying the NO_(x) storage and reduction catalyst 12 on theparticulate filter 12 a in this way, the partition walls of the exhaustgas inflow passages 60 and the exhaust gas outflow passages 61, that is,the two surfaces of the partition walls 64 and the walls inside theholes in the partition walls 64, carry a catalyst carrier comprised offor example alumina. FIGS. 4(A) and (B) schematically showcross-sections of the surface part of this catalyst carrier 45. As shownin FIGS. 4(A) and (B), the surface of the catalyst carrier 45 carries aprecious metal catalyst 46 dispersed on them. Further, the surface ofthe catalyst carrier 45 is formed with a layer of an NO_(x) absorbent47.

In this embodiment according to the present invention, platinum Pt isused as the precious metal catalyst 46, while as the ingredient formingthe NO_(x) absorbent 47, at least one element for example selected frompotassium K, sodium Na, cesium Cs, or another such alkali metal, bariumBa, calcium Ca, or another such alkali earth, and lanthanum La, yttriumY, or another such rare earth is used.

If referring to the ratio of the air and fuel (hydrocarbons) suppliedinto the engine intake passage, combustion chamber 2, and exhaustpassage upstream of the NO_(x) storage and reduction catalyst 12 as the“air-fuel ratio of the exhaust gas”, the NO_(x) absorbent 47 performsthe NO_(x) absorption/release action of absorbing the NO_(x) when theair-fuel ratio of the exhaust gas is lean and releasing the absorbedNO_(x) when the oxygen concentration in the exhaust gas falls.

That is, explaining the case of use of barium Ba as an ingredientforming the NO_(x) absorbent 47 as an example, when the air-fuel ratioof the exhaust gas is lean, that is, when the exhaust gas has a highoxygen concentration, the NO contained in the exhaust gas, as shown inFIG. 4(A), is oxidized on the platinum Pt 46 to become NO₂, then isabsorbed in the NO_(x) absorbent 47 and bonds with the barium oxide BaOwhile dispersing in the form of nitrate ions NO₃ ⁻ into the NO_(x)absorbent 47. In this way, NO_(x) is absorbed in the NO_(x) absorbent47. So long as the oxygen concentration in the exhaust gas is high, NO₂is formed on the surface of the platinum Pt 46. So long as the NO_(x)absorption capability of the NO_(x) absorbent 47 is not saturated, theNO₂ is absorbed in the NO_(x) absorbent 47 and nitrate ions NO₃ ⁻ aregenerated.

As opposed to this, if the air-fuel ratio of the exhaust gas is maderich or the stoichiometric air-fuel ratio, the oxygen concentration inthe exhaust gas falls, so the reaction proceeds in the oppositedirection (NO₃ ⁻→NO₂) and therefore, as shown in FIG. 4(B), the nitrateions NO₃ ⁻ in the NO_(x) absorbent 47 are released in the form of NO₂from the NO_(x) absorbent 47. Next, the released NO_(x) is reduced bythe unburned HC and CO contained in the exhaust gas.

When the air-fuel ratio of the exhaust gas is lean in this way, that is,when combustion occurs under a lean air-fuel ratio, the NO_(x) in theexhaust gas is absorbed in the NO_(x) absorbent 47. However, ifcombustion is performed continuously under a lean air-fuel ratio, theNO_(x) absorption capability of the NO_(x) absorbent 47 ends up becomingsaturated. Therefore, NO_(x) ends up no longer being able to be absorbedby the NO_(x) absorbent 47. Therefore, in the embodiment according tothe present invention, fuel is added from the fuel addition valve 14before the absorption capability of the NO_(x) absorbent 47 becomessaturated so as to make the air-fuel ratio of the exhaust gastemporarily rich and thereby make the NO_(x) absorbent 47 release theNO_(x).

Next, the oxidation catalyst 11 arranged at the upstream side of theNO_(x) storage and reduction catalyst 12 in FIG. 1 will be explained.

FIG. 5 shows a side cross-sectional view of the oxidation catalyst 11.As shown in FIG. 5, the oxidation catalyst 11 forms a honeycombstructure which is provided with a plurality of exhaust gas passages 65extending straight. The substrate of this oxidation catalyst 11 iscomprised of alumina, zirconia, or another complex oxide. FIGS. 6(A) and(B) show cross-sections of the surface part of the substrate of theoxidation catalyst 11. As shown in FIGS. 6(A) and (B), the surface ofthe substrate 50 carries the platinum Pt shown by 51 and the palladiumPd shown by 52 dispersed on it.

Platinum Pt has the property of trapping oxygen on its surface, but theamount of oxygen which can be trapped is small. As opposed to this,palladium Pd has the ability to trap a far larger amount of oxygen thanplatinum Pt. Therefore, when the air-fuel ratio of the exhaust gas islean, as shown in FIG. 6(A), a far greater amount of oxygen is trappedand stored on the palladium Pd compared with the platinum Pt. On theother hand, if viewed from the point of the oxidizing ability, platinumPt has a much stronger oxidizing ability, while the oxidizing ability ofpalladium Pd is weak. In this way, platinum Pt and palladium Pd differconsiderably in properties.

Further, as explained above, if adding fuel from the fuel addition valve14 to make the air-fuel ratio of the exhaust gas rich, the NO_(x)absorbent 47 releases NO_(x) and the released NO_(x) is reduced by theunburned HC and CO contained in the exhaust gas. In this case, if theadded fuel is liquid in state, even if stoichiometrically the air-fuelratio of the exhaust gas becomes rich, the oxygen concentration in theexhaust gas will not fall, so the NO_(x) absorbent 47 will not releaseNO_(x). However, in the invention of the present application, even ifthe added fuel is liquid in state, the NO_(x) absorbent 47 can be madeto release the NO_(x) well.

That is, part of the fuel added from the fuel addition valve 14 becomesa gas, but the majority of the fuel flows in the form of drops throughthe exhaust passage together with the exhaust gas, then the fuel dropsdeposit on the oxidation catalyst 11. As a result, as shown in FIG.6(B), the platinum Pt and palladium Pd are covered by the fuel drops 53.If the platinum Pt is covered by the fuel drops 53, the oxygen includedin the exhaust gas is blocked by the deposited fuel drops 53 and cannotreach the surface of the platinum Pt. Therefore, if taking note of onlythe platinum Pt, no matter how strong the oxidizing ability of theplatinum Pt, the oxidation reaction of the fuel drops 53 will notproceed that much and therefore the fuel drops 53 will not vaporize thatmuch.

As opposed to this, the palladium Pd stores a large amount of oxygen onits surface, so if the palladium Pd is covered by the fuel drops 53, thefuel drops 53 will be oxidized by the larger amount of oxygen on thepalladium Pd. At this time, a large amount of heat of oxidation reactionis produced. This heat of oxidation reaction causes the fuel drops 53covering the palladium Pd of course and also the fuel drops 53 coveringthe platinum Pt to be vaporized. If the fuel drops 53 covering theplatinum Pt are vaporized, the oxygen in the exhaust gas can reach thesurface of the platinum Pt and as a result the oxidation reaction of theunburned HC and CO on the platinum Pt becomes brisk. As a result, theoxygen concentration in the exhaust gas falls, so the NO_(x) absorbent47 releases NO_(x) and the vaporized unburned HC and CO reduce thereleased NO_(x).

By having the oxidation catalyst 11 carry not only platinum Pt, but alsopalladium Pd in this way, the NO_(x) absorbent 47 can be made to releaseand reduce the NO_(x). However, when the sum of the platinum Pt andpalladium Pd is fixed, if increasing the amount of palladium Pd anddecreasing the amount of platinum Pt, the vaporization of the fuel drops53 on the platinum Pt is promoted by the heat of oxidation reaction ofthe stored oxygen of the palladium Pd, but since the amount of platinumPt is small, the unburned HC and CO are not sufficiently oxidized and,as a result, an excellent NO_(x) release action cannot be obtained.

As opposed to this, if reducing the amount of the palladium Pd andincreasing the amount of the platinum Pt, the vaporization of the fueldrops 53 on the platinum Pt by the heat of oxidation reaction of thestored oxygen of the palladium Pd is not sufficiently promoted, so evenif increasing the amount of platinum Pt, sufficient amounts of unburnedHC and CO are not oxidized, therefore, in this case as well, anexcellent NO_(x) release action cannot be obtained. That is, the ratioof platinum Pt and palladium Pd giving an excellent NO_(x) releaseaction exists in a ratio of suitable ratios neither extremely large norextremely small

FIG. 7 shows experimental results showing the relationship between theoxidation speed showing the amount of oxidation per unit time and theratio of the number of moles of platinum Pt with respect to the sum ofthe number of moles of the platinum Pt and the number of moles of thepalladium Pd (hereinafter referred to as the “platinum molar ratio”). InFIG. 7, the higher the oxidation speed, the better the action of releaseof the NO_(x) from the NO_(x) absorbent 47, therefore, as shown in FIG.7, the NO_(x) release action becomes the best when the platinum molarratio is about 66 percent.

FIG. 8 shows experimental results showing the relationship between theNO_(x) purification rate after the end of the NO_(x) release action andthe temperature Tc of the oxidation catalyst 11. Note that in FIG. 8,the black dots show the case where the oxidation catalyst 11 carriesonly platinum Pt, that is, the case where the platinum molar ratio is100 percent, while the white dots show the case where the platinum molarratio is 66 percent. Both when the platinum molar ratio is 100 percentand is 66 percent, as the temperature Tc of the oxidation catalyst 11becomes higher, the NO_(x) purification rate becomes higher, but nomatter what the temperature Tc, it is learned that the NO_(x)purification rate is higher when the platinum molar ratio is 66 percentthan when the platinum molar ratio is 100 percent.

FIG. 9 shows the relationship between the NO_(x) purification rate andplatinum molar ratio when the temperature Tc of the oxidation catalyst11 is about 350° C. in FIG. 8. The pattern of change of the NO_(x)purification rate shown in FIG. 9 has the same trend as the pattern ofchange of the oxidation speed shown in FIG. 7. As shown in FIG. 9, theNO_(x) purification rate becomes highest when the platinum molar ratiois about 66 percent. Therefore, the amounts of the platinum Pt andpalladium Pd to be carried on the oxidation catalyst 11 can be said tobe most preferably determined so that the platinum molar ratio becomesabout 66 percent.

Note that even if the NO_(x) purification rate becomes lower than themaximum NO_(x) purification rate, if the degree of drop is about 5percent, it still can be said that the rate is about the maximum NO_(x)purification rate. The range of the platinum molar ratio where theNO_(x) purification rate can be said to be about the maximum NO_(x)purification rate in this way becomes between about 58 percent to about75 percent as shown by X in FIG. 9. Therefore, the platinum molar ratiocan be said to be preferably set between about 58 percent to about 75percent.

Note that NO_(x) purification rate can be practically used even if about10 percent lower than the maximum NO_(x) purification rate. The range ofthe platinum molar ratio where the NO_(x) purification rate becomes 10percent lower than the maximum NO_(x) purification rate becomes, asshown by Y in FIG. 9, between about 50 percent to about 80 percent.Therefore, when viewed practically, the platinum molar ratio should beset between about 50 percent to 80 percent.

In the embodiment shown in FIG. 2, the catalyst carrier 45 of the NO_(x)storage and reduction catalyst 12 (FIG. 4) carries not only platinum Ptbut also palladium Pd. The NO_(x) purification rate in this case alsobecomes about the same as the NO_(x) purification rate shown in FIG. 9.Therefore, in this case as well, the NO_(x) purification rate becomeshighest when the platinum molar ratio is about 66 percent. Therefore,the amounts of the platinum Pt and palladium Pd to be carried on theNO_(x) storage and reduction catalyst 12 can be said to be mostpreferably determined so that the platinum molar ratio becomes about 66percent.

Further, in the embodiment shown in FIG. 2 as well, the range of theplatinum molar ratio where the NO_(x) purification rate can be said tobe about the maximum NO_(x) purification rate is between about 58percent to about 75 percent. Therefore, in this embodiment as well, itcan be said that the platinum molar ratio is preferably set to about 58percent to about 75 percent.

Further, in the embodiment shown in FIG. 2 as well, the range of theplatinum molar ratio where the NO_(x) purification rate becomes 10percent lower than the maximum NO_(x) purification rate, that is, therange of the platinum molar ratio which can be used when viewedpractically, becomes about 50 percent to about 80 percent, therefore, inthis embodiment as well, when viewed from a practical perspective, theplatinum molar ratio may be set between about 50 percent to 80 percent.

FIG. 10 shows the NO_(x) concentration in the exhaust gas flowing outfrom the NO_(x) storage and reduction catalyst 12 in the case where forexample, in a gasoline engine, the air-fuel ratio in the combustionchamber is made rich, that is, the air-fuel ratio of the exhaust gasitself forming the gas state is made rich, for the case where theoxidation catalyst 11 carries only platinum Pt and the case where itcarries platinum Pt and palladium Pd. As will be understood from FIG.10, the NO_(x) concentration when the air-fuel ratio of the exhaust gasshould be made rich is lower when the oxidation catalyst 11 carries onlyplatinum Pt, but becomes higher when the oxidation catalyst 11 carriesplatinum Pt and palladium Pd.

That is, if the air-fuel ratio of the exhaust gas is made rich, theoxygen concentration in the exhaust gas falls, so the NO_(x) absorbent47 releases NO_(x). However, if the oxidation catalyst 11 carriespalladium Pd, the unburned HC and CO contained in the exhaust gas areoxidized by the large amount of oxygen stored on the palladium Pd andthe released NO_(x) can no longer be reduced by the unburned HC and CO.As a result, when the oxidation catalyst 11 carries platinum Pt andpalladium Pd, the NO_(x) concentration becomes higher.

That is, when the NO_(x) absorbent 47 should release NO_(x), when theair-fuel ratio of the exhaust gas is made rich in the gas state, if theoxidation catalyst 11 is made to carry palladium Pd, a large amount ofNO_(x) is exhausted into the atmosphere and therefore the NO_(x)purification rate ends up falling. However, even if using the sameoxidation catalyst 11 carrying palladium Pd, when the NO_(x) absorbent47 should release NO_(x), a high NO_(x) purification rate is obtainedopposite to the case where fuel is added to the exhaust gas in the formof drops. If viewed from the point of the purification of NO_(x) in thisway, the palladium Pd acts effectively when the fuel is added in theform of drops.

Next, the NO_(x) release control will be explained with reference toFIG. 11.

FIG. 11 shows the changes in the NO_(x) amount ΣNOX stored in the NO_(x)storage and reduction catalyst 12 and the timing of adding fuel forrelease of NO_(x) to make the air-fuel ratio A/F of the exhaust gasrich. The NO_(x) amount exhausted from the engine per unit time changesin accordance with the engine operating state. Therefore, the NO_(x)amount stored in the NO_(x) storage and reduction catalyst 12 per unittime also changes in accordance with the engine operating state. In theembodiment according to the present invention, the NO_(x) amount NOXAstored in NO_(x) storage and reduction catalyst 12 per unit time isstored as a function of the required torque TQ and engine rotationalspeed N in the form of the map shown in FIG. 12 in advance in the ROM32. By cumulatively adding this NO_(x) amount NOXA, the NO_(x) amountΣNOX stored in the NO_(x) storage and reduction catalyst 12 iscalculated.

On the other hand, in FIG. 11, MAX indicates the maximum NO_(x) storageamount which the NO_(x) storage and reduction catalyst 12 can store,while NX indicates the allowable value of the NO_(x) amount which theNO_(x) storage and reduction catalyst 12 can be made to store.Therefore, as shown in FIG. 11, if the NO_(x) amount ΣNOX reaches theallowable value NX, fuel is added, whereby the air-fuel ratio A/F of theexhaust gas flowing into the NO_(x) storage and reduction catalyst 12 istemporarily made rich and therefore the NO_(x) storage and reductioncatalyst 12 releases NO_(x).

On the other hand, the particulate matter contained in the exhaust gasis trapped and successively oxidized on the particulate filter 12 acarrying the NO_(x) storage and reduction catalyst 12. However, if theamount of the particulate matter deposited becomes greater than theparticulate matter oxidized, the particulate matter will gradually buildup on the particulate filter 12 a. In this case, if the deposited amountof the particulate matter increases, a drop in the engine output willend up being caused. Therefore, when the deposited amount of theparticulate matter increases, the deposited particulate matter must beremoved. In this case, if raising the temperature of the particulatefilter 12 a under an excess of air to about 600° C., the depositedparticulate matter is removed by oxidation.

Therefore, in the embodiment according to the present invention, whenthe amount of the particulate matter deposited on the particulate filter12 a exceeds the allowable amount, the temperature of the particulatefilter 12 a is raised under a lean air-fuel ratio of the exhaust gas,whereby the deposited particulate matter is removed by oxidation.Specifically speaking, in the embodiment according to the presentinvention, when the differential pressure ΔP before and after theparticulate filter 12 a detected by the differential pressure sensor 21exceeds the allowable value PX, it is judged that the amount of thedeposited particulate matter has exceeded the allowable amount. At thistime, the air-fuel ratio of the exhaust gas flowing into the particulatefilter 12 a is maintained lean and fuel is added from the fuel additionvalve 14 to raise the temperature of the particulate filter 12 a by theheat of oxidation reaction of this added fuel.

FIG. 13 shows an exhaust purification processing routine.

Referring to FIG. 13, first, at step 100, the NO_(x) amount NOXA storedper unit time is calculated from the map shown in FIG. 12. Next, at step101, this NOXA is added to the NO_(x) amount ΣNOX stored in the NO_(x)storage and reduction catalyst 12. Next, at step 102, whether the storedNO_(x) amount ΣNOX has exceeded the allowable value NX is judged. WhenΣNOX>NX, the routine proceeds to step 103, where processing for additionof fuel from the fuel addition valve 14 is performed. Next, at step 104,the differential pressure sensor 21 detects the differential pressure ΔPbefore and after the particulate filter 12 a. Next, at step 105, whetherthe differential pressure ΔP has exceeded the allowable value PX isjudged. When ΔP>PX, the routine proceeds to step 106, where temperatureelevation control of the particulate filter 12 a is performed.

FIGS. 14(A) and (B) show another embodiment.

If the carrier carrying the palladium Pd is made to contain an elementof a basicity able to give electrons to the palladium Pd, a large numberof electrons will gather on the surface of the palladium Pd. If a largenumber of electrons gather on the surface of the palladium Pd in thisway, a large amount of the oxygen contained in the exhaust gas will beadsorbed on the surface of the palladium Pd in search of electrons,therefore the oxygen storage amount of the palladium Pd will increase.Therefore, in the embodiment shown in FIGS. 14(A) and (B), to increasethe oxygen storage amount of the palladium Pd, the carrier carrying thepalladium Pd may be made to contain an element of a basicity able togive electrons to the palladium Pd, for example, an alkali earth or rareearth element.

That is, FIG. 14(A), like FIG. 6(A), shows the cross-section of thesurface part of the substrate 50 of the oxidation catalyst 11. In thisembodiment, as shown in FIG. 14(A), the substrate 50 is made to includethe rare earth element lanthanum La. In this embodiment, this lanthanumLa functions as a thermal stabilizer of the substrate 50. On the otherhand, FIG. 14(B) shows an embodiment in the case where the oxidationcatalyst 11 is comprised of an aggregate of the particulate carrier. Inthis embodiment, the particulate carrier is comprised of a basic carrier70 including an alkali earth or rare earth element and an acidic carrier71 including tungsten or titanium or another element with a highelectric cathodicity. In addition to platinum Pt, palladium Pd isselectively carried on the basic carrier 70.

As explained above, according to the present invention, by setting themolar ratio of the platinum Pt with respect to the sum of the platinumPt and palladium Pd carried on the oxidation catalyst or NO_(x) storageand reduction catalyst between about 50 percent to about 80 percent, itis possible to secure an excellent NO_(x) release action from the NO_(x)storage and reduction catalyst.

1. An internal combustion engine providing a fuel addition device adding fuel in the form of atomized drops inside an engine exhaust passage, providing an oxidation catalyst inside the engine exhaust passage downstream of the fuel addition device, providing an NO_(x) storage and reduction catalyst storing the NO_(x) contained in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and releasing the stored NO_(x) when the air-fuel ratio of the inflowing exhaust gas becomes a stoichiometric air-fuel ratio or rich inside the engine exhaust passage downstream of the oxidation catalyst, adding fuel from the fuel addition device when making the air-fuel ratio of the exhaust gas flowing into the NO_(x) storage and reduction catalyst rich so as to make the NO_(x) storage and reduction catalyst release NO_(x), and making the fuel added at this time deposit in the form of drops on the oxidation catalyst, wherein to make the fuel deposited in the form of drops on the oxidation catalyst vaporize, the oxidation catalyst carries as a precious metal not only platinum Pt, but also palladium Pd and the molar ratio of the platinum Pt with respect to the sum of the platinum Pt and palladium Pd carried on the oxidation catalyst is set from about 50 percent to about 80 percent.
 2. An exhaust purification system of internal combustion engine as set forth in claim 1, wherein a molar ratio of platinum Pt with respect to the sum of the platinum Pt and palladium Pd carried on the oxidation catalyst is set to about 58 percent to about 75 percent.
 3. An exhaust purification system of internal combustion engine as set forth in claim 1, wherein a carrier of the oxidation catalyst is comprised of a basic carrier including at least one element selected from an alkali earth or rare earth.
 4. An exhaust purification system of internal combustion engine as set forth in claim 1, wherein the NO_(x) storage and reduction catalyst is carried on a particulate filter.
 5. An internal combustion engine providing a fuel addition device adding fuel in the form of atomized drops inside an engine exhaust passage, providing an NO_(x) storage and reduction catalyst storing the NO_(x) contained in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and releasing the stored NO_(x) when the air-fuel ratio of the inflowing exhaust gas becomes a stoichiometric air-fuel ratio or rich inside the engine exhaust passage downstream of the oxidation catalyst, adding fuel from the fuel addition device when making the air-fuel ratio of the exhaust gas flowing into the NO_(x) storage and reduction catalyst rich so as to make the NO_(x) storage and reduction catalyst release NO_(x), and making the fuel added at this time deposit in the form of drops on the NO_(x) storage and reduction catalyst, wherein to make the fuel deposited in the form of drops on the NO_(x) storage and reduction catalyst vaporize, the NO_(x) storage and reduction catalyst carries as a precious metal not only platinum Pt, but also palladium Pd and the molar ratio of the platinum Pt with respect to the sum of the platinum Pt and palladium Pd carried on the NO_(x) storage and reduction catalyst is set from about 50 percent to about 80 percent.
 6. An exhaust purification system of internal combustion engine as set forth in claim 5, wherein a molar ratio of platinum Pt with respect to the sum of the platinum Pt and palladium Pd carried on the NO_(x) storage and reduction catalyst is set to about 58 percent to about 75 percent.
 7. An exhaust purification system of internal combustion engine as set forth in claim 5, wherein the NO_(x) storage and reduction catalyst is carried on a particulate filter. 